1. Field of the Invention
The present invention relates to a method and device for manufacturing an optical fiber secondary preform made by progressively collapsing an over-cladding tube on an optical fiber primary preform.
2. Description of the Prior Art
An optical fiber primary preform is manufactured by a well known process such as a modified chemical vapor deposition. One modified chemical vapor deposition process is disclosed by John B. MacChesney, et al.,in U.S. Pat. No. 4,217,027 entitled Optical Fiber Fabrication And Resulting Product. The optical fiber primary preform should have the volume ratio of a core to a clad within a given limitation. To raise the efficiency of production, the over-cladding tube collapses on the optical fiber primary preform, which over-cladding tube is made of glass with a prescribed optical and geometrical specialty. Further, the optical fiber primary preform is placed within the over-cladding tube. Then, the optical fiber secondary preform is produced by collapsing the over-cladding tube on the optical fiber primary preform.
A conventional method for producing such optical fiber secondary preform is disclosed in U.S. Pat. No. 4,668,263 to Hirosi Yokota, et al. entitled Method For Producing Glass Preform For Optical Fiber. The optical fiber secondary preform is produced by collapsing the over-cladding tube on the optical fiber primary preform. The conventional method for producing such optical fiber secondary preform is shown in FIG. 1. In the construction of FIG. 1, the over-cladding tube 32 engages with chucks 30 of a glass lathe, a turning connecter 40 is joined with one side of the over-cladding tube 32, and a gas suck hole 38 is set up in the turning connecter 40. A vacuum pump 46 is set up in the gas suck hole 38 by a valve connection 42. The optical fiber primary preform 34 is set up within the over-cladding tube 32 by a supporter 36. Under this structure, as shown in FIG. 2, one end side of the over-cladding tube 32 is sealed up by using a burner 44, then the temperature influenced by the burner 44 is maintained as over nineteen hundred degrees centigrade. The burner 44 is moved along a circumference of the over-cladding tube 32 in order to collapse the over-cladding tube 32 on the optical fiber primary preform 34. Then the optical fiber secondary preform is produced.
In such conventional method for producing the optical fiber secondary preform, problems still remain. Since the optical fiber primary preform receives more heat than the over-cladding tube receives, as the heating time is long in a procedure to seal up one end of the over-cladding tube, there is a problem that a concentricity error becomes large by the melt. There is further problem that the thicker the thickness of the over-cladding tube is the more difficult it is to seal. Further, there is much material loss, since the part engaged with the chucks of the glass lathe or the non-collapsed part should be cut off as useless parts after producing the optical fiber secondary preform, which is produced by the collapse of the over-cladding tube on the optical fiber primary preform.
The Method For Producing A Single Mode Fiber Preform disclosed in U.S. Pat. No. 4,596,589 to Gregory A. Perry uses supporting elements different from Yokota, et al. Perry employs a handle to which the core or primary preform is symmetrically adhered at one end thereof, and the handle is chucked into the tailstock of a lathe. A barrier or over-cladding tube is slid over the primary preform until one end of the over-cladding tube aligns with the adhered end of the primary preform. A tack weld to weld the over-cladding tube to the primary preform is made near to the headend of the lathe, and the ends of the primary preform and over-cladding tube adjacent the headend of the lathe are chucked to the headend so that the work piece can be rotated while heating the over-cladding tube in order to collapse the over-cladding tube onto the primary preform.
There is a problem in Perry""s method wherein the heating is a chance of the longitudinal center of the over-cladding tube is off-center from the longitudinal center of the primary preform resulting in a large concentricity error by the melt.
It is therefore, an object of the present invention to provide a method and device for over cladding an optical fiber primary preform which reduces concentricity error of the core when collapsing an over-cladding tube on an optical fiber primary preform.
It is another object of the present invention to provide a method and device for over cladding the optical fiber primary preform which can reduce raw material loss.
It is still another object of the present invention to provide a method and device for over cladding the optical fiber primary preform which is easily sealed up one end side of the over-cladding tube.
In accordance with the present invention for achieving these objects, one end side of the optical fiber primary preform adheres to a hand bar in a longitudinal direction for supporting the optical fiber primary preform. The hand bar has a sealing-up part on a circumference part of the hand bar for sealing up one end side of the over-cladding tube. The purity of the hand bar is different from that of the over-cladding tube. A supporting handle tube for supporting another end side of the over-cladding tube adheres to the another end side of the over-cladding tube in the longitudinal direction. A quartz ring is inserted into an inner diameter of the supporting handle tube which has low purity in order to fix the longitudinal center, or axis, of the optical fiber primary preform concentrically with the longitudinal center of the over-cladding tube. The optical fiber primary preform is inserted into the inner diameter of the ring and a part of an optical fiber primary preform supporter, ie. part of the hand bar, is sealed up in the inner diameter of the over-cladding tube. With a predetermined collapse condition, the over-cladding tube collapses on the optical fiber primary preform by heat during rotation.